Postnatal development of efferent synapses in the rat cochlea

The development of olivocochlear efferent axons and their contacts in the p ostnatal cochlea was studied after DiI applications to the olivocochlear bu ndle in the ipsilateral brainstem of rats from 0 to 10 days of age (P0-10). Light microscopic analyses showed that labeled axons reached the vicinity of inner hair cells by PO and outer hair cells by P2. Electron microscopic analyses demonstrated that labeled immature efferent axons are present amon g supporting cells of the greater epithelial ridge as well as inner hair ce lls at PO. The first efferent contacts that contacted inner hair cells cont ained a few irregularly sized vesicles and, occasionally, mitochondria. Pos tsynaptic specializations within inner hair cells apposed to labeled effere nt axons included subsynaptic cisterns, irregularly sized vesicles, and syn aptic bodies. Similar features were present in unlabeled profiles, presumed to be afferents, indicating that immature efferent axons could not be reli ably distinguished from afferents without positive labeling. Efferent axons synapsed with outer hair cells by P4 and had synapse-like contacts at the bases of Deiters' cells at P4 and P6. Contacts between afferents and effere nts were observed frequently in the inner spiral bundle from P6. As they ma tured, efferent axon terminals contacting hair cells contained increasing n umbers of synaptic vesicles and were typically apposed by well-defined post synaptic cisterns, thus acquiring distinctive profiles. (C) 2000 Wiley-Liss , Inc.